The invention relates to a method for regulating transmit power in a radio system. The invention is suitable in particular for use in a third-generation mobile radio system, but is not restricted to mobile radio systems.
The most widespread mobile radio system is GSM (global system for mobile communications), which was developed for a single service, i.e. voice transmission. The GSM mobile radio system is referred to as a 2nd-generation system.
In contrast to this, a plurality of services, which are intended to be transmitted within one transmission protocol via commonly used physical channels, are provided for its successor, the 3rd mobile radio generation, which is currently being standardized in Europe under the name UMTS (universal system for mobile communications).
The standardization documents ETSI SMG2/UMTS L23 expert group, Tdoc SMG2 UMTS-L23 357/98, dated 6.10.1998, Tdoc SMG2 508/98 and Tdoc SMG2 515/98, dated 16.11.1998, provide an overview of the current state of development of standardization and, in particular, of the requirements describing how a transmission protocol can support the transport of data of a plurality of services.
The use of a common physical channel for the transmission of data of a plurality of services requires a unique mapping rule to specify the allocation of the services to different segments of the physical channel. A physical channel is defined, for example, by a frequency band, a spread code (CDMA code division multiple access) and, if necessary, a time slot within a frame.
The following terms are used to describe the mapping rule:
Transport Format (TF):
A transport format defines a data rate, a coding, an interleaving, a data rate adaptation through punctuation and an error-protection rule of a transport channel for a service.
Transport Format Set (TFS):
This refers to a set of possible transport formats, which are permitted for a special service.
Transport Format Combination (TFC):
This term indicates a possible combination of transport formats of the different services which are mapped onto a common physical channel.
Transport Format Combination Set (TFCS):
This refers to a set of possible TFCs as a subset of all TFCs which are permitted for a special connection.
Transport Format Combination Identifier (TFCI):
This information indicates the currently used combination of transport formats within the TFCS.
Examples of the transport formats can be found in ETSI SMG2/UMTS L23 expert group, Tdoc SMG2 UMTS-L23 357/98, dated Jun. 10, 1998, pp. 14-16.
TFC modifiability and therefore regular signaling of the TFCI are required for user-oriented selection of the currently used combination of transport formats of the different services.
CDMA separation of the different channels is used in 3rd-generation mobile radio transmission systems. In order to achieve different data rates with the resources available on the air interface, the spreading of the data which are to be transmitted is variable. In order to achieve flexible spreading of this type, for example, either redundant data bits can be added as part of the channel coding, or data bits can be removed (“repetition”=expansion or “puncturing”=compression of data). Flexible spreading is also achieved by modifying the spreading factors of the CDMA spread codes which are used. If the current TFC is modified, this normally results in a modification of the spreading of the data to be transmitted. “Rate matching” can be achieved through the different spreading of the data, i.e. the available channel capacity is optimally utilized by the data to be transmitted.
In order to guarantee transmission quality which is as constant as possible (determined e.g. as the bit error rate or signal-to-noise or signal-to-interference ratio) at the minimum transmit power, feedback power regulation is required, in which the receiver transfers information to the transmitter relating to the required transmit power regulation using the signals received by it. The feedback from the receiver to the transmitter relating to the required adaptation of the transmit power is performed through corresponding signaling, referred to as the “TPC” (Transmit Power Control) bits. To do this, the receiver evaluates the useful signals received from the transmitter, for example, by evaluating the signal-to-noise ratio as a quality parameter of the receive signal.
In a favorable manner, the receiver could obtain the value of this quality parameter required for the respective TFC which is used from the TFCI transferred to it by the transmitter. In the FDD (Frequency Division Duplex) variant of UMTS, the complete TFCI is received at the end of a transmission frame. In the TDD (Time Division Duplex) variant of UMTS, the TFCI is transferred in each time slot. However, particularly with the use of a “joint detector”, which performs simultaneous detection of all the spread codes which are used in the radio cell, a certain time passes following receipt of the TFCI before the TFCI is fully decoded. As a result, the receiver may not start the evaluation of the TPC bits for the current time frame until relatively late. The earliest possible time for transfer to the transmitter of the TPC bits determined by the receiver is thus also delayed. In the case of TDD, this results in a minimum spacing, which must not be understepped, between the two transmission directions (uplink and downlink). This signifies a restriction in the channel allocation for the transmission directions.
In J-H Wen et al, “Performance of Short-Term Fading Prediction-Based Power Control Method for DS-CDMA Cellular Mobile Radio Networks”, a method is described which takes into account the short-term fading of the transmission signal when regulating the transmit power of a base station.
In DE 198 21 519 A1, the ratio of receive power to noise power is determined in the receiver of a mobile part, is compared with a set value and a signaling bit is transmitted to a base station depending on the result of the comparison. The signaling bit is then used to calculate the current or updated transmit power.
In U.S. Pat. No. 5,987,333, a multichannel communications system is described in which, using an iterative method, the transmit power is monitored for each channel in such a way that a predefined minimal signal-to-noise plus signal-to-interference ratio (SNIR) is attained for the channels.